Magnesium (MG) Alloy and Method of Producing Same

ABSTRACT

Embodiments of a magnesium (Mg) alloy and method for producing the same are disclosed. One such embodiment, among others, is a method for producing a magnesium (Mg) alloy, comprising the steps of: (a) producing a Mg powder aggregate by mixing Mg powder and at least one strengthening agent, the strengthening agent selected from: a carbon, a metal, and a combination thereof; (b) agglomerating the aggregate; and (c) sintering the agglomerated aggregate to produce the Mg alloy. Preferably, although not necessarily, steps (a) and (b) are performed using a ball mill. Moreover, the strengthening agent may be, for example but not limited to, carbon nanotubes, copper, tin, titanium, or silicon carbide. The resulting Mg alloy comprises nano-scale crystalline and/or micro-scale crystalline lattice structures and a yield strength that is at least as high as steel, exhibiting a yield strength that is about 320 MPa to 500 MPa.

CLAIM OF PRIORITY

This application claims priority to and the benefit of provisionalapplication No. 61/668,132, filed on Jul. 5, 2012, which is incorporatedherein by reference in its entirety.

BACKGROUND

Traditional cast and wrought magnesium (Mg) alloys have been verylimited in their usage in connection with automotive structuralapplications and other structural applications due to poor performanceat cold or even warm forming conditions, caused by low ductility and lowfracture toughness. Although Mg is much lighter than steel (Fe3C) andaluminum (Al) as one of the prominent candidates for lightweightmaterial design, the strength level of Mg is still lower than steel andaluminum and therefore undesirable for use in connection with automotiveand other structural applications.

SUMMARY OF THE INVENTION

Embodiments of a new magnesium (Mg) alloy and method for producing thesame are disclosed herein.

One such embodiment, among others, comprises a method for producing amagnesium (Mg) alloy, comprising the steps of: (a) producing a Mg powderaggregate by mixing Mg powder and at least one strengthening agent, thestrengthening agent selected from the group consisting of: carbon,metal, and a combination thereof; (b) agglomerating the Mg powderaggregate; and (c) sintering the agglomerated aggregate to produce theMg alloy. Preferably, although not necessarily, steps (a) and (b) areperformed using a ball mill. Moreover, the strengthening agent may be,for example but not limited to, carbon nanotubes, copper, tin, titanium,or silicon carbide. The resulting Mg alloy comprises nano-scalecrystalline lattice structures, micro-scale crystalline latticestructures, or both, and exhibits a yield strength that is at least ashigh as steel. The strength of the resulting Mg alloy can be altered anddesigned with the amount of the strengthening agent mixed. The yieldstrength is about 320 MPa to 500 MPa.

Another embodiment, among others, comprises a method for producing a Mgalloy, comprising the steps of: (a) mixing and agglomerating in a ballmill a Mg powder aggregate having a Mg powder and at least onestrengthening agent, the strengthening agent selected from the groupconsisting of: carbon, metal, and a combination thereof; and (b)sintering the agglomerated aggregate to produce the Mg alloy. Moreover,the strengthening agent may be, for example but not limited to, carbonnanotubes, copper, or titanium. The resulting Mg alloy comprisesnano-scale crystalline lattice structures, micro-scale crystallinelattice structures, or both, and exhibits a yield strength that is atleast as high as steel.

Other embodiments, methods, apparatus, features, and advantages of thepresent disclosure will be or become apparent to one with skill in theart upon examination of the following detailed description. It isintended that all such additional embodiments, methods, features, andadvantages be included within this description, be within the scope ofthe present disclosure, and be protected by the accompanying claims.

DETAILED DESCRIPTION

It is desirable to improve both ductility and strength levels of Mgalloys by innovative manufacturing methods in which the grain size issignificantly reduced, the grain boundary bounding resistance (GBenergy) improved, and the dislocation/twinning behavior favorable todeformation. The present disclosure includes an innovative manufacturingmethod to produce highly ductile and high strength Mg alloys.

The method involves synthesizing Mg metal in its powder form, mixed withone or more strengthening agents by high-energy deformation processes.The strengthening agents can be (a) carbon-based, such as but notlimited to, carbon (C) nanotubes or (b) metallic-based, such as but notlimited to, copper (Cu) or titanium (Ti). In some embodiments, themetals have a hexagonal close-packed crystal structure. Elementalpowders of a purity of about 99.9% or better and a particle size ofabout −120 mesh are used for producing samples.

Next, the aggregate of the Mg powder and the strengthening agent areagglomerated using any suitable apparatus. In the preferred embodiment,the Mg powder and the strengthening agent are mixed in and agglomeratedin a conventional ball mill. Preferably, although not necessarily, theadded agents are ductile.

The ball milling is conducted in a standard lab-scale shaker mill withhardened steel balls and vial. To avoid oxidation, the powders aresealed in a container under a noble-gas environment (i.e., nitrogen (N₂)or other inert gases), opened after a period of up to about 24 hours ofball milling, and subsequently put in a case for subsequent coldpressing and elevated temperature sintering. The vial temperature duringball milling process is kept constant by gas cooling. All operationsconcerning the Mg samples were carried out without exposure to air. Theoptical microscopy of the powder in an early stage of milling showeddeformation by twinning and re-twinning within the grains, developingsub-grain boundaries, which eventually defined nanometre-sized grains.The grain size reduction examined using X-Ray Diffraction (XRD) revealeda rapid decrease and then saturation of the grain size at the nanometerlevel.

After a number of hours of high-energy ball milling, the aggregate isprocessed by high temperature sintering (heating) at particulartemperature-time histories to further enhance ductility and fracturetoughness.

Optionally, after ball milling the Mg mixture and prior to sintering,the mixture can be processed in a compaction device to shape it.

After sintering, the samples were subjected to X-ray and TransmissionElectron Microscope (TEM) analysis in order to quantify themicrostructural state. In addition, multi-scale modeling simulationswere performed to understand the mechanisms of mixing Mg powders withductile and strengthening agents.

The ductility and strength of the Mg alloy can be tailored and designedby the control of the volumetric fraction of ductile and strengtheningagents added to the Mg powders and the associated processing parameters,such as the duration of ball milling, the sintering temperature, theduration of sintering, and the sintering pressure level. The strength ofthe resulting Mg alloy can be altered and designed with the amount ofthe strengthening agent mixed, and the yield strength will be in therange of about 320 MPa to 500 MPa.

There are many applications for the Mg alloy and the method for makingsame. One application of the method is that it has proven to be aneffective processing technique for producing nanocrystalline and/ormicrocrystalline metal allloys. Nanocrystalline and/or microcrystallinemetal alloys have shown improved chemical, physical and mechanicalproperties due to the ultra-fine grain structure and the high volume ofgrain boundaries. Mg is a hexagonal closed pack (HCP) metal with amelting point of about 923 degrees K, which is similar to that ofaluminum. The major attraction of Mg is the low density, which gives itshigh specific strength. At room temperature, plastic deformation of Mgis limited to basal slip and twinning, and hence, the ductility of Mg isrelatively low (about −14% compared to about −25% of aluminum intension). However, the ductility of Mg can be improved by suppressingtwin formation through grain refinement, and because finer grainsreduced the grain boundary back stresses allowing easier accommodationof grain boundary sliding and rotation.

Another application of the enhanced Mg alloys produced by the method ofthe present disclosure is used for storage (housing) of hydrogen (H₂)due to the high hydrogen storage capacity, the low cost and weight of Mgpowders. The hydrogen storage properties of MgH₂ are significantlyenhanced by a proper engineering of the microstructure and surface. Ballmilling, which is used for fabrication of nanocrystalline and/ormicrocrystalline Mg alloy, improves both the morphology of the powdersand the surface activity for hydrogenation. Mg alloys can be produced ina nanocrystalline and/or microcrystalline form by mixing tin (Sn) and/orsilicon carbide (SiC) powders with Mg powders, which gives remarkableimprovement of absorption/desorption kinetics. The hydriding propertiesare further enhanced by catalysis through nano-particles of palladium(Pd) located on Mg surface. Nanocrystalline and/or microcrystalline Mgalloy with such a catalyst exhibits an outstanding hydrogenationperformance: very fast kinetics, operation at lower temperatures thanconventional Mg and no need for activation.

Other applications of the Mg alloy are, for example but not limited to,the manufacture of automotive parts, sporting goods (such as footballhelmets and football helmet facemasks, etc.), etc.

It should be further noted that mechanical milling of the abovedescribed method also improves the corrosion resistance of Mg in passiveconditions (KOH solution) as well as in more active corrosion conditions(borate solution). A Mg oxide (MgO) enrichment in the milled powders,which seems to be essentially related to the powder oxidation during themilling, contributes to the improved corrosion resistance of milled Mg,since MgO is thermodynamically much less reactive than Mg in thepresence of water. Additional investigations, such as TEM or AFM/STMcharacterizations, are useful to investigate the improved surface layersof milled powders.

It should be emphasized that the above-described embodiments of thepresent disclosure, particularly, any “preferred” embodiments, aremerely possible non-limiting examples of implementations, merely setforth for a clear understanding of the principles of the disclosure.Many variations and modifications may be made to the above-describedembodiment(s) of the disclosure without departing substantially from thespirit and principles of the disclosure. All such modifications andvariations are intended to be included herein within the scope of thepresent disclosure.

At least the following is claimed:
 1. A method for producing a magnesium(Mg) alloy, comprising the steps of: producing a Mg powder aggregate bymixing Mg powder and at least one strengthening agent, the strengtheningagent selected from the group consisting of: a carbon, a metal, and acombination thereof; agglomerating the aggregate; and sintering theagglomerated aggregate to produce the Mg alloy.
 2. The method of claim1, wherein the producing and agglomerating steps are performed in a ballmill.
 3. The method of claim 1, wherein the agent is at least oneselected from the group consisting of: carbon nanotubes, copper, tin,titanium, silicon carbide, and a combination thereof.
 4. The method ofclaim 1, wherein the agent is a metal with a hexagonal close-packedcrystal structure.
 5. The method of claim 1, further comprising the stepof compacting the agglomerated aggregate prior to performing thesintering step.
 6. The method of claim 1, wherein during theagglomerating step, the aggregate is sealed in a container with an inertgas in order to prevent oxidation of the aggregate.
 7. A magnesium (Mg)alloy produced by the method of claim
 1. 8. A method for producing amagnesium (Mg) alloy, comprising the steps of: mixing and agglomeratingin a ball mill a Mg powder aggregate having a Mg powder and at least onestrengthening agent, the strengthening agent selected from the groupconsisting of: a carbon, a metal, and a combination thereof; andsintering the agglomerated aggregate to produce the Mg alloy.
 9. Themethod of claim 8, further comprising the step of preventing oxidationof the aggregate during the agglomerating step by introducing an inertgas into the ball mill.
 10. The method of claim 8, wherein the agent isat least one selected from the group consisting of: carbon nanotubes,copper, tin, titanium, silicon carbide, and a combination thereof.
 11. Amagnesium (Mg) alloy comprising a structure selected from the groupconsisting of: a nano-scale crystalline lattice structure, a micro-scalecrystalline lattice structure, and a combination thereof; and exhibitinga yield strength that is about 320 MPa to 500 MPa.
 12. The Mg alloy ofclaim 11, wherein the alloy comprises Mg and a strengthening agent, andwherein the strengthening agent is at least one selected from the groupconsisting of: a carbon, a metal, and a combination thereof.
 13. The Mgalloy of claim 12, wherein the agent is a metal with a hexagonalclose-packed crystal structure.
 14. The Mg alloy of claim 11, whereinthe alloy comprises Mg and a strengthening agent, and wherein thestrengthening agent is at least one selected from the group consistingof: carbon nanotubes, copper, tin, titanium, silicon carbide, and acombination thereof.